Shearing is made much use of in the production of the metal members used in automobiles, household electric appliances, building structures, ships, bridges, construction machines, various plants, penstocks, etc. FIGS. 1A and 1B schematically show modes of shearing. FIG. 1A schematically shows a mode of shearing for forming a hole in the workpiece, while FIG. 1B schematically shows a mode of shearing for forming an open section in the workpiece.
In the shearing operation shown in FIG. 1A, a workpiece 1 is arranged on a die 3, a punch 2 is pushed inward in the downward direction 2a, that is, the sheet thickness direction of the workpiece 1, to form a hole in the workpiece 1. In the shearing operation shown in FIG. 1B, the workpiece 1 is arranged on the die 3 and, similarly, the punch 2 is pushed inward in the downward direction 2a, that is, the sheet thickness direction of the workpiece 1, to form an open section in the workpiece 1.
A sheared edge 9 of a worked material 10 formed by a shearing operation usually, as shown in FIG. 2, is comprised of a shear droop 4, burnished surface 5, fracture surface 6, and burr 7. The shear droop 4 is formed at a surface 8a of a top part of the worked material 10 due to the workpiece 1 being pushed inward by the punch. The burnished surface 5 is formed by the workpiece 1 being locally stretched due to the workpiece 1 being pulled inward at the clearance between the punch and die. The fracture surface 6 is formed by the workpiece 1 pulled into the clearance between the punch and die breaking. The burr 7 is formed at a surface 8b of a bottom part of the worked material 10 when the workpiece 1 pulled into the clearance between the punch and die breaks and separates from the worked material 10.
The sheared edge is in general inferior in surface properties compared with the worked surface formed by machining. For example, it has the problems that the hydrogen embrittlement resistance is low, the fatigue strength is low, or strength flange cracking (cracking occurring at sheared edge due to press-forming after shearing) easily occurs. In particular, in high strength steel sheet, hydrogen embrittlement cracking and a drop in the fatigue strength easily occur due to tensile residual stress.
Various arts have been proposed for solving the problems with the sheared edge. These arts generally can be divided into ones which modify the structures of the punch and die to improve the fatigue strength, stretch flangeability, and other surface properties of the sheared edge (for example, see PLTs 1 to 3) and ones which treat the sheared edge by coining, shaving, etc. to improve the hydrogen embrittlement resistance, fatigue strength, and other surface properties of the sheared edge (for example, see PLTs 4 to 8).
However, with the arts of modifying the structures of the punch and die, there are limits to the improvement of the surface properties of the sheared edge, while with the art of treating the sheared edge, the productivity falls and the manufacturing costs rise by the amount of the increase of one process.